diff --git a/CMakeLists.txt b/CMakeLists.txt
index 67a3bd8..d2ad461 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -46,6 +46,8 @@ add_library(dom STATIC
   src/ftxui/dom/blink.cpp
   src/ftxui/dom/bold.cpp
   src/ftxui/dom/border.cpp
+  src/ftxui/dom/box_helper.cpp
+  src/ftxui/dom/box_helper.hpp
   src/ftxui/dom/clear_under.cpp
   src/ftxui/dom/color.cpp
   src/ftxui/dom/composite_decorator.cpp
diff --git a/src/ftxui/dom/box_helper.cpp b/src/ftxui/dom/box_helper.cpp
new file mode 100644
index 0000000..d90da2a
--- /dev/null
+++ b/src/ftxui/dom/box_helper.cpp
@@ -0,0 +1,85 @@
+#include "ftxui/dom/box_helper.hpp"
+
+namespace ftxui {
+namespace box_helper {
+
+namespace {
+// Called when the size allowed is greater than the requested size. This
+// distributes the extra spaces toward the flexible elements, in relative
+// proportions.
+void ComputeGrow(std::vector<Element>* elements,
+                 int extra_space,
+                 int flex_grow_sum) {
+  for (Element& element : *elements) {
+    int added_space =
+        extra_space * element.flex_grow / std::max(flex_grow_sum, 1);
+    extra_space -= added_space;
+    flex_grow_sum -= element.flex_grow;
+    element.size = element.min_size + added_space;
+  }
+}
+
+// Called when the size allowed is lower than the requested size, and the
+// shrinkable element can absorbe the (negative) extra_space. This distribute
+// the extra_space toward those.
+void ComputeShrinkEasy(std::vector<Element>* elements,
+                       int extra_space,
+                       int flex_shrink_sum) {
+  for (Element& element : *elements) {
+    int added_space = extra_space * element.min_size * element.flex_shrink /
+                      std::max(flex_shrink_sum, 1);
+    extra_space -= added_space;
+    flex_shrink_sum -= element.flex_shrink * element.min_size;
+    element.size = element.min_size + added_space;
+  }
+}
+
+// Called when the size allowed is lower than the requested size, and the
+// shrinkable element can not absorbe the (negative) extra_space. This assign
+// zero to shrinkable elements and distribute the remaining (negative)
+// extra_space toward the other non shrinkable elements.
+void ComputeShrinkHard(std::vector<Element>* elements,
+                       int extra_space,
+                       int size) {
+  for (Element& element : *elements) {
+    if (element.flex_shrink) {
+      element.size = 0;
+      continue;
+    }
+
+    int added_space = extra_space * element.min_size / std::max(1, size);
+    extra_space -= added_space;
+    size -= element.min_size;
+
+    element.size = element.min_size + added_space;
+  }
+}
+
+}  // namespace
+
+void Compute(std::vector<Element>* elements, int target_size) {
+  int size = 0;
+  int flex_grow_sum = 0;
+  int flex_shrink_sum = 0;
+  int flex_shrink_size = 0;
+
+  for (auto& element : *elements) {
+    flex_grow_sum += element.flex_grow;
+    flex_shrink_sum += element.min_size * element.flex_shrink;
+    if (element.flex_shrink)
+      flex_shrink_size += element.min_size;
+    size += element.min_size;
+  }
+
+  int extra_space = target_size - size;
+  if (extra_space >= 0)
+    ComputeGrow(elements, extra_space, flex_grow_sum);
+  else if (flex_shrink_size + extra_space >= 0)
+    ComputeShrinkEasy(elements, extra_space, flex_shrink_sum);
+  else
+    ComputeShrinkHard(elements, extra_space + flex_shrink_size,
+                      size - flex_shrink_size);
+}
+
+}  // namespace box_helper
+}  // namespace ftxui
diff --git a/src/ftxui/dom/box_helper.hpp b/src/ftxui/dom/box_helper.hpp
new file mode 100644
index 0000000..9e0ebda
--- /dev/null
+++ b/src/ftxui/dom/box_helper.hpp
@@ -0,0 +1,24 @@
+#ifndef FTXUI_DOM_BOX_HELPER_HPP
+#define FTXUI_DOM_BOX_HELPER_HPP
+
+#include <vector>
+
+namespace ftxui {
+namespace box_helper {
+
+struct Element {
+  // Input:
+  int min_size = 0;
+  int flex_grow = 0;
+  int flex_shrink = 0;
+
+  // Output;
+  int size = 0;
+};
+
+void Compute(std::vector<Element>* elements, int target_size);
+
+}  // namespace box_helper
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_DOM_BOX_HELPER_HPP */
diff --git a/src/ftxui/dom/hbox.cpp b/src/ftxui/dom/hbox.cpp
index 092e38f..55bc115 100644
--- a/src/ftxui/dom/hbox.cpp
+++ b/src/ftxui/dom/hbox.cpp
@@ -3,6 +3,7 @@
 #include <utility>    // for move
 #include <vector>     // for vector
 
+#include "ftxui/dom/box_helper.hpp"   // for Box
 #include "ftxui/dom/elements.hpp"     // for Element, Elements, hbox
 #include "ftxui/dom/node.hpp"         // for Node
 #include "ftxui/dom/requirement.hpp"  // for Requirement
@@ -38,92 +39,23 @@ class HBox : public Node {
   void SetBox(Box box) override {
     Node::SetBox(box);
 
-    int space = box.x_max - box.x_min + 1;
-    int extra_space = space - requirement_.min_x;
-
-    int size = 0;
-    int flex_grow_sum = 0;
-    int flex_shrink_sum = 0;
-    int flex_shrink_size = 0;
-    for (auto& child : children_) {
-      const Requirement& r = child->requirement();
-      flex_grow_sum += r.flex_grow_x;
-      flex_shrink_sum += r.min_x * r.flex_shrink_x;
-      if (r.flex_shrink_x) {
-        flex_shrink_size += r.min_x;
-      }
-      size += r.min_x;
+    std::vector<box_helper::Element> elements(children_.size());
+    for (size_t i = 0; i < children_.size(); ++i) {
+      auto& element = elements[i];
+      const auto& requirement = children_[i]->requirement();
+      element.min_size = requirement.min_x;
+      element.flex_grow = requirement.flex_grow_x;
+      element.flex_shrink = requirement.flex_shrink_x;
     }
+    int target_size = box.x_max - box.x_min + 1;
+    box_helper::Compute(&elements, target_size);
 
-    if (extra_space >= 0)
-      SetBoxGrow(box, extra_space, flex_grow_sum);
-    else if (flex_shrink_size + extra_space >= 0)
-      SetBoxShrinkEasy(box, extra_space, flex_shrink_sum);
-    else
-      SetBoxShrinkHard(box, extra_space + flex_shrink_size,
-                       size - flex_shrink_size);
-  }
-
-  void SetBoxGrow(Box box, int extra_space, int flex_grow_sum) {
     int x = box.x_min;
-    for (auto& child : children_) {
-      Box child_box = box;
-      const Requirement& r = child->requirement();
-
-      int added_space =
-          extra_space * r.flex_grow_x / std::max(flex_grow_sum, 1);
-      extra_space -= added_space;
-      flex_grow_sum -= r.flex_grow_x;
-
-      child_box.x_min = x;
-      child_box.x_max = x + r.min_x + added_space - 1;
-
-      child->SetBox(child_box);
-      x = child_box.x_max + 1;
-    }
-  }
-
-  void SetBoxShrinkEasy(Box box, int extra_space, int flex_shrink_sum) {
-    int x = box.x_min;
-    for (auto& child : children_) {
-      Box child_box = box;
-      const Requirement& r = child->requirement();
-
-      int added_space = extra_space * r.min_x * r.flex_shrink_x /
-                        std::max(flex_shrink_sum, 1);
-      extra_space -= added_space;
-      flex_shrink_sum -= r.flex_shrink_x * r.min_x;
-
-      child_box.x_min = x;
-      child_box.x_max = x + r.min_x + added_space - 1;
-
-      child->SetBox(child_box);
-      x = child_box.x_max + 1;
-    }
-  }
-
-  void SetBoxShrinkHard(Box box, int extra_space, int size) {
-    int x = box.x_min;
-    for (auto& child : children_) {
-      Box child_box = box;
-      const Requirement& r = child->requirement();
-
-      if (r.flex_shrink_x) {
-        child_box.x_min = x;
-        child_box.x_max = x - 1;
-        child->SetBox(child_box);
-        continue;
-      }
-
-      int added_space = extra_space * r.min_x / std::max(1, size);
-      extra_space -= added_space;
-      size -= r.min_x;
-
-      child_box.x_min = x;
-      child_box.x_max = x + r.min_x + added_space - 1;
-
-      child->SetBox(child_box);
-      x = child_box.x_max + 1;
+    for (size_t i = 0; i < children_.size(); ++i) {
+      box.x_min = x;
+      box.x_max = x + elements[i].size - 1;
+      children_[i]->SetBox(box);
+      x = box.x_max + 1;
     }
   }
 };
diff --git a/src/ftxui/dom/vbox.cpp b/src/ftxui/dom/vbox.cpp
index 1ed02d8..6dd26e7 100644
--- a/src/ftxui/dom/vbox.cpp
+++ b/src/ftxui/dom/vbox.cpp
@@ -3,6 +3,7 @@
 #include <utility>    // for move
 #include <vector>     // for vector
 
+#include "ftxui/dom/box_helper.hpp"  // for Box
 #include "ftxui/dom/elements.hpp"     // for Element, Elements, vbox
 #include "ftxui/dom/node.hpp"         // for Node
 #include "ftxui/dom/requirement.hpp"  // for Requirement
@@ -38,92 +39,23 @@ class VBox : public Node {
   void SetBox(Box box) override {
     Node::SetBox(box);
 
-    int space = box.y_max - box.y_min + 1;
-    int extra_space = space - requirement_.min_y;
-
-    int size = 0;
-    int flex_grow_sum = 0;
-    int flex_shrink_sum = 0;
-    int flex_shrink_size = 0;
-    for (auto& child : children_) {
-      const Requirement& r = child->requirement();
-      flex_grow_sum += r.flex_grow_y;
-      flex_shrink_sum += r.min_y * r.flex_shrink_y;
-      if (r.flex_shrink_y) {
-        flex_shrink_size += r.min_y;
-      }
-      size += r.min_y;
+    std::vector<box_helper::Element> elements(children_.size());
+    for (size_t i = 0; i < children_.size(); ++i) {
+      auto& element = elements[i];
+      const auto& requirement = children_[i]->requirement();
+      element.min_size = requirement.min_y;
+      element.flex_grow = requirement.flex_grow_y;
+      element.flex_shrink = requirement.flex_shrink_y;
     }
+    int target_size = box.y_max - box.y_min + 1;
+    box_helper::Compute(&elements, target_size);
 
-    if (extra_space >= 0)
-      SetBoxGrow(box, extra_space, flex_grow_sum);
-    else if (flex_shrink_size + extra_space >= 0)
-      SetBoxShrinkEasy(box, extra_space, flex_shrink_sum);
-    else
-      SetBoxShrinkHard(box, extra_space + flex_shrink_size,
-                       size - flex_shrink_size);
-  }
-
-  void SetBoxGrow(Box box, int extra_space, int flex_grow_sum) {
-    int y = box.y_min;
-    for (auto& child : children_) {
-      Box child_box = box;
-      const Requirement& r = child->requirement();
-
-      int added_space =
-          extra_space * r.flex_grow_y / std::max(flex_grow_sum, 1);
-      extra_space -= added_space;
-      flex_grow_sum -= r.flex_grow_y;
-
-      child_box.y_min = y;
-      child_box.y_max = y + r.min_y + added_space - 1;
-
-      child->SetBox(child_box);
-      y = child_box.y_max + 1;
-    }
-  }
-
-  void SetBoxShrinkEasy(Box box, int extra_space, int flex_shrink_sum) {
-    int y = box.y_min;
-    for (auto& child : children_) {
-      Box child_box = box;
-      const Requirement& r = child->requirement();
-
-      int added_space = extra_space * r.min_y * r.flex_shrink_y /
-                        std::max(flex_shrink_sum, 1);
-      extra_space -= added_space;
-      flex_shrink_sum -= r.flex_shrink_y * r.min_y;
-
-      child_box.y_min = y;
-      child_box.y_max = y + r.min_y + added_space - 1;
-
-      child->SetBox(child_box);
-      y = child_box.y_max + 1;
-    }
-  }
-
-  void SetBoxShrinkHard(Box box, int extra_space, int size) {
-    int y = box.y_min;
-    for (auto& child : children_) {
-      Box child_box = box;
-      const Requirement& r = child->requirement();
-
-      if (r.flex_shrink_y) {
-        child_box.y_min = y;
-        child_box.y_max = y - 1;
-        child->SetBox(child_box);
-        continue;
-      }
-
-      int added_space = extra_space * r.min_y / std::max(1, size);
-      extra_space -= added_space;
-      size -= r.min_y;
-
-      child_box.y_min = y;
-      child_box.y_max = y + r.min_y + added_space - 1;
-
-      child->SetBox(child_box);
-      y = child_box.y_max + 1;
+    int x = box.x_min;
+    for (size_t i = 0; i < children_.size(); ++i) {
+      box.y_min = x;
+      box.y_max = x + elements[i].size - 1;
+      children_[i]->SetBox(box);
+      x = box.y_max + 1;
     }
   }
 };